This invention generally relates to rotary vane motors, and is specifically concerned with a rotary vane motor for effectively extracting mechanical energy from an expanding, cryogenic gas at low rotational speeds.
Rotary vane motors are known in the prior art. Such motors typically comprise a housing having a cylindrical interior, and a cylindrical rotor eccentrically mounted in the interior of the housing. The rotor includes a plurality of uniformly spaced, radially oriented slots for slidably receiving a plurality of rectangularly shaped vanes. Both the housing and the rotor are typically formed of metal. The eccentric placement of the rotor within the cylindrical enclosure defined by the housing leaves a gap between the rotor and the housing that is crescent-shaped in cross section. In operation, pressurized fluid (usually compressed air) is admitted in an inlet port in the housing located at one of the narrow ends of the crescent-shaped gap. The pressurized fluid pushes against the trailing faces of the slidable vanes, thereby rotating the rotor. Centrifugal force radially slings the vanes out of their slots such that their outer edges sealingly engage the inner surface of the housing. The vanes reciprocate in their respective slots as their outer edges sealingly and slidably engage the interior surface of the housing. The pressurized fluid is expelled out an outlet port located at the other end of the crescent-shaped gap.
Such prior art rotary-vane motors are well adapted for powering tools such as pneumatic wrenches and grinders where the operating speeds of the motor shaft are greater than 2000 rpm, and where a pressurized drive fluid in the form of a supply of compressed and lubricant-containing air is plentifully and cheaply supplied by the shop air compressor. While there is a certain loss of efficiency in such designs due to the leakage (or "blow-by") of compressed air between the sides of the rotating vanes and the sidewalls of the housing, the inefficiencies created by such blow-by are relatively small as a percentage of the overall air mass that flows through the motor at speeds of 2000 rpm or greater. Moreover, the entrained oil or other lubricant typically present in the shop air used to drive such motors keeps the internal friction of the motor down to a useable level.
The applicants have observed, however, that such prior art motor designs are not well suited for use at relatively low rotational speeds (i.e., under 1500 rpm), and under conditions where the drive fluid contains no lubricant or moisture, and is cryogenically generated. Such an application for a rotary vane motor may occur, for example, in a cryogenic refrigeration system powered by a tank of liquified carbon dioxide, such as that disclosed in co-pending Ser. No. 08/501,372, filed Jul. 12, 1995, also assigned to the Thermo King Corporation of Minneapolis, Minn. In such an application, the motor is used to drive an evaporator blower and an alternator to recharge the battery that powers the refrigeration control system, and low rotational speeds are preferred to enhance the efficiency of the fan blades of the blower. Because lower volumes of compressed gas are passed through the motor housing at lower speeds below 2000 rpm, the blow-by of gas between the sides of the rotor and the sidewalls of the housing can result in a 20% or greater loss of efficiency in prior art designs, where efficiency is defined as the ability of the motor to convert the energy of the compressed gas into rotary power. Additionally, such prior art motors cannot begin to operate efficiently without the lubricant that is normally present in compressed shop air. While the use of vanes formed from self-lubricating plastic material can ameliorate the frictional problems encountered when the pressurized gas contains no lubricant, the relatively light weight of such vanes can create a sealing problem at low rpm rates, since the centrifugal force that tends to sling the vane into engagement against the inner surface of the housing may not be of sufficient magnitude to create an effective sealing engagement between the vane and the housing interior. Finally, such prior art air motors are not well designed to operate under extremes of temperature which can occur, for example, when the drive gas originates from a cryogen such as liquid carbon dioxide. When such a drive gas is used, the internal components of the motor may be subjected to temperature extremes ranging from -100.degree. F. to +130.degree. F., depending upon the ambient temperature. Under such conditions, the applicants have observed that even if the vanes, the rotor slots, and the internal dimensions of the enclosure are carefully dimensioned in order to minimize inefficiencies caused by blow-by, such dimensioning does not hold up over such a broad range of temperature extremes due to the different thermal coefficients of expansions of the different materials forming these components. Consequently, either binding or excessive slack occurs between the vanes, the rotor slots, and the housing.
Clearly, there is a need for a rotary vane type motor that is capable of efficiently running at low rpm in order to drive certain types of blowers and other devices which operate best at low rpms. Ideally, such a motor would have both a minimum amount of blow-by and a minimum amount of friction during operation. Finally, the internal components of such a motor should continue to accurately interfit and cooperate with one another over a broad range of temperature extremes in the event that a cryogenic gas is used as the drive fluid.
The foregoing illustrates limitations known to exist in present devices and methods. Thus it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.